The invention is used in applications in very many technical domains, for single-carrier and for multi-carrier signals, particularly for amplitude modulations in quadrature (MAQ) regardless of the number of states. It is particularly advantageous for transmission in burst mode.
Systems developed in telecommunications operate at increasingly high frequencies, using modulations with a very large number of states. The quality of the local oscillator that controls the frequency transposition then becomes a technological lock. As the frequency of these systems increases, it becomes technologically more difficult to design oscillators with good frequency stability and low phase noise.
Therefore an attempt is made to optimise performances of the phase locking loop to overcome problems due to degradation to system performances induced by hyperfrequency oscillators of the type available to the general public.
In general, demodulation consists of putting received values into a space taking account of the modulation constellation used. This space is broken down into decision regions, defined by decision-making boundaries. Each region is assigned to one of the constellation states which is considered to be the most probable, and that is selected as the demodulation result when a received value is located in this region.
In the following, we will present examples of an MAQ modulation received in a single-sensor receiver. The reception space is then the Fresnel plane (I/Q plane). This two-dimensional example efficiently describes the state-of-the-art and the characteristics of the invention. However, it is quite clear that the invention is equally applicable to other modulation types, possibly using spaces with more than two dimensions. Similarly, the use of multi-sensor receivers can lead to the definition of reception spaces with more than two dimensions.
Therefore, MAQ type digital modulation techniques are based on the use of a modulation constellation in single-sensor receivers, conventionally represented in the I/Q plane in the form shown in FIG. 1 in the special case of an MAQ16 modulation (only the first quadrant is shown. The three other quadrants are directly deduced by symmetry).
The modulation points 11 are uniformly distributed at equal distances from each other. The modulation then consists of choosing one of the points 14 from among the 16 points available in the constellation. The received value 12 after transmission through a transmission channel affected by various disturbances is often significantly offset (13) from the ideal point 14.
Therefore, the demodulation operation consists of associating the received value 12 with the most probable emitted point 14. This is done by defining demodulation boundaries 15 parallel to the I and Q axes, maximising the distances (the received value 12 is considered to correspond to the closest point 14). Therefore, these boundaries 15 define zones 16, each associated with a point 14 in the modulation constellation.
This technique provides a relatively efficient means of overcoming Gaussian additive noise. On the other hand, errors can occur in the presence of important phase errors, as is the case particularly in the synchronisation system latching phase in the presence of a frequency offset, or in the presence of loud phase noise. For example, a phase shift 17 will lead to a demodulation error, the received value 18 being considered to correspond to point 19 and not to point 14.
In particular, one purpose of the invention is to overcome this disadvantage in the state of the art.
More precisely, one purpose of the invention is to provide a demodulation technique for reducing the effects of a frequency offset more efficiently than is possible with a conventional technique.
Consequently, one purpose of the invention is to provide such a technique enabling faster latching of the synchronisation system, particularly in the presence of a frequency offset.
Obviously, another purpose is to provide such a technique for reducing the probability of the synchronisation system becoming unlatched.
Another purpose of the invention is to provide such a technique that is easy and inexpensive to implement, particularly in receivers used by the general public without needing to make any modifications to hyper-frequency oscillators.
Another purpose of one particular aspect of the invention is to provide such a technique that is adaptive, and that takes account of all disturbances induced through the transmission channel (phase noise or Gaussian additive noise).
These objectives, and others that will become clearer later, are achieved using a method for demodulation of a digital signal received through a transmission channel, comprising a step in which each received value of the said received signal is associated with a corresponding point in the modulation constellation, as a function of decision-making boundaries, plotted as a function of at least one phase and/or amplitude characteristic of the said modulation, so as to associate a corresponding decision region with each of the said points in the constellation.